The present invention relates to a toner for developing an electrostatic latent image, to a toner container containing such a toner and to an apparatus for forming images such as a copying machine, a printing machine or a facsimile machine of a single color-, multicolor- or full color-type.
The present day offices are flooded with office electric appliances such as personal computers, printers, copiers, scanners, and facsimile machines. Documents including text documents, graphs etc. are created using personal computers. Moreover, occasion where such documents are printed in color is increasing. Many of the images output by the printers are solid, line, or halftone images. Marketing needs for the image quality are changing accordingly and needs such as high reliability are increasing.
A developer used in an image forming method such as electrophotography, electrostatic recording or electrostatic printing is deposited onto a latent image bearing member, typically a photoconductor. The developer is transferred from the photoconductor to a transfer medium such as a transfer paper and fixed on a surface thereof. Known developers include a two-component developer composed of a carrier and a toner and a one-component developer containing no carrier.
In case of the two-component developer, the carrier is used for charging and transporting the developer in a stable manner. The two-component developer, however, has a problem because the toner particles gradually adhere on the surface of the carrier. Moreover, the concentration of the toner in the developer gradually decreases as only the toner is consumed. Moreover, since the ratio of the toner and the carrier in the developer has to be kept constant, there arises a problem that size of the apparatus becomes large.
The one-component developer is free from the above problems and, therefore, the size of the apparatus can be compact. Hence, the one-component developer is popular and widely used in present day developing systems. The one-component developer can be classified into two types; i.e. a magnetic toner and a non-magnetic toner.
The magnetic toner includes a magnetic material such as magnetite and is held on a developer carrier such as a developing sleeve having a magnet provided therein. The magnetic toner forms a layer on the sleeve with the thickness thereof being adjusted by a suitably thickness control member such as a blade or a roller. The magnetic toner is practically increasingly used recently for small-size printers.
The non-magnetic toner of the one-component developer, on the other hand, is supported on a developing sleeve by electrostatic force. Thus, the non-magnetic toner is supplied to the substrate by being pressed with a supply roller to form a layer thereon. The thickness of the toner layer is adjusted by a suitable thickness control member such as a blade or a roller. Since the non-magnetic toner does not contain a magnetic material which has unavoidably a color, there is obtainable an advantage over the magnetic developer that the non-magnetic toner can be suitably used for color image formation. Moreover, the image forming apparatus that uses the non-magnetic developer does not require any magnet and can be made light-weight and compact and, therefore, is widely used as full color printers.
However, the one-component developer has a lot of problems to be solved. In particular, charging and transportation failures tend to occur when image formation is continuously repeated for a considerably long period of time at a high speed. Namely, as described above, the one-component developer, after it is transported onto the developing sleeve, forms a thin layer by means of the thickness controlling member and is contacted with the latent image on the photoconductor. At that time, contact between the toner and the developing sleeve and between the toner and the thickness controlling member is only for a very short period of time. Therefore, a time for which the toner is charged by friction is very short. As a result, in contrast to the two-component development system using the carrier, more of the toner tends to have a low or opposite charge in the one-component development system. In the non-magnetic one-component system particularly, the toner is transported typically by means of at least one toner transport member. It is known that the thickness of the toner layer on the toner transport member surface must be as thin as possible. When the toner layer is thick, only a portion near the surface of the toner layer is charged and it becomes difficult to evenly charge the whole toner layer. Thus, it is necessary that the toner should be quickly charged to provide the desired level of charging amount.
It is also important that the material constituting the toner supplying member, thickness controlling member and developing sleeve should be suitably selected to provide the one-component, non-magnetic toner with sufficient electric charges. Further, the non-magnetic toner should be forcibly pressed by toner supplying member and thickness controlling member against the developing sleeve so as to sufficiently charge the toner. Under such conditions, however, the toner is heated by friction to cause xe2x80x9ctoner filmingxe2x80x9d as a result of melt-adhesion of the toner, in particular the binder resin thereof, on the developing sleeve. The filming results in shortening of the service life of the developing sleeve as well as unstable chargeability of the toner. In addition, the mechanical impact applied to the toner causes grinding thereof, which results in reduction of the density of the images as well as formation of white spots in the images due to failure of proper toner transportation on the sleeve. Thus, the image forming machine is unable to withstand usage over a long period of service, and there is a problem that an image formation unit called a xe2x80x9cprocess cartridgexe2x80x9d has to be replaced at an early stage such as every few-thousand copies.
Thermal characteristics of toner, especially a binder resin thereof, also play an important role in image formation. A press heating fixation method has been generally adopted for fixing a toner image on an image receiving sheet such as paper. In such a method, the image-bearing sheet is brought into pressure contact with a heat roll for fixing the image thereon. Since the heating efficiency is high, the fixation of image can be carried out at a high speed. To further improve the fixation speed, it is necessary that the toner should be fixed at a lower temperature, i.e. the softening point of the binder resin of the toner should be low.
At this time, when the temperature of the heat roll is excessively high, the toner is excessively melted and adhered to the heat roll and further transferred to a succeeding transfer sheet (hot offset). For the purpose of preventing the hot offset, an attempt has been made to use a heat roll having a surface made of a releasing property and to apply a releasing agent such as silicone oil to the surface of the heat roll. This method is effective to prevent hot offset when the toner used has low temperature fixability. However, such a toner has a problem because the storage stability thereof is low.
For the production of a full color image, at least three color toner images (cyan, magenta and yellow toner images and, if necessary, a black toner image) are successively formed on a transfer sheet and the superimposed images are fixed simultaneously. Thus, the thickness of the fixed image is unavoidably large. In order to produce clear color images, therefore, a toner for use in full color image forming is required to provide an image with suitable gloss. Further, in order to prevent formation of cracks or delamination of the image, the toner is required to provide an image having suitable fixing strengths.
Properties of the binder resin constituting a toner are main factors of fixation characteristics of the toner. Styrene-acrylate copolymer resins, polyester resins and polyol resins are generally used as a binder resin. It is known that styrene-acrylate copolymer resins are inferior to polyester resins and polyol resins with respect to low temperature fixability and image strength, whereas the polyester resins and polyol resins are apt to cause hot offset. To cope with the hot offset problem of the polyester resins and polyol resins, proposals have been made to increase surface cohesive force thereof in the molten state by increasing the polymerization degree thereof, by introducing crosslinkages thereinto or by introducing gel components. However, these methods adversely affect not only the low temperature fixability but also the pulverizability.
With regard to pulverizability, to meet with increasing requirement for high quality and good half-tone gradation of images, the particle diameters of toners becomes smaller and smaller. Thus, it is important that toner should be produced with good pulverization efficiency. When polyester resins and polyol resins whose molecular cohesive force has been increased are used, a difficulty is caused during pulverization for the production of toners. Further, since the finer the toner particles, the greater becomes the Van der Waal""s force, the use of a binder resin having a high cohesive force causes aggregation of the particles and adhesion thereof onto surfaces of a mill. Such aggregation also results in lowering of classification efficiency and an increase of the production costs.
A thought might occur to one ordinary skilled in the art that the pulverizability can be improved by using a binder resin having a reduced molecular weight. However, a reduction of the molecular weight of a resin results in a decrease of the glass transition point thereof, which in turn causes deterioration of the storage stability, anti-blocking property, image strength and anti-offset property of the toner.
Japanese Laid Open Patent Publication No. H2-269364 discloses the use of a polyester resin containing, as a diol component, a polyoxyethylene or polyoxypropylene adduct of bisphenol A as a binder resin of a toner. When such a polyester is used for a full color toner, the image is apt to crack. Further, the low temperature fixability and storage stability of the toner are not satisfactory.
In attempting to improve low temperature fixability of toner, several methods in which two different polyester resins having different properties are used in a toner have been proposed. For example, a method in which a non-linear polyester resin is used in combination with a linear polyester resin (Japanese Laid Open Patent Publication No. S60-90344); a method in which a crosslinkable polyester having a glass transition point (Tg) not lower than 50xc2x0 C. and a softening point not higher than 200xc2x0 C. is used in combination with a linear polyester resin having a softening point not higher than 150xc2x0 C. and a weight average molecular weight (MW) of from 3,000 to 50,000 (Japanese Laid Open Patent Publication No. S64-15755); a method in which a non-linear polyester polymer having a weight average molecular weight not less than 5,000 and a variance ratio (MW/MN) not less than 20 is used in combination with a non-linear polyester polymer having a weight average molecular weight of from 1,000 to 5,000 and a variance ratio not less than 4 (Japanese Laid Open Patent Publication No. H02-82267); a method in which an organic metal compound including a linear polyester resin having an acid value of from 5 to 60 and a non-linear polyester resin having an acid value less than 5 is included in a toner (Japanese Laid Open Patent Publication No. H3-229264); and a method in which a first saturated polyester resin is used in combination with a second saturated polyester resin having an acid value 1.5 or more times the acid value of the first polyester resin (Japanese Laid Open Patent Publication No. H03-41470). Although these binder resins provide good fixation characteristics, such as low temperature fixability, the pulverizability is not fully satisfactory. Further, the known binder resins are ill-suited for use in a one-component type, non-magnetic toner.
It is, therefore, an object of the present invention to provide a toner suitably used as a one-component, non-magnetic toner for developing an electrostatic latent image.
Another object of the present invention is to provide a toner of the above-mentioned type which shows excellent service life, pulverizability and fixability.
It is a further object of the present invention to provide a toner of the above-mentioned type which can produce high quality color image in a stable manner, without causing background stains or toner filming, for a long period of service.
It is yet a further object of the present invention to provide a toner of the above-mentioned type which permits the use of an image forming apparatus in which toner replenishment can be done by exchange of a toner cartridge rather than exchange of a process cartridge.
It is yet a further object of the present invention to provide a toner cartridge, an image forming method and an image forming apparatus using the above toner.
In accomplishing the foregoing objects, there is provided in accordance with one aspect of the present invention a toner for developing an electrostatic latent image, comprising a binder resin and a colorant, wherein said binder resin comprises different two resins (A) and (B),
said resin (A) being substantially free of tetrahydrofuran insolubles, being selected from the group consisting of polyester resins and polyol resins and having such a molecular weight distribution according to gel permeation chromatography that (a) a main peak is present in a molecular weight of 3000 to 9,000 and (b) that portion of said resin (A) having a molecular weight of 500 or less accounts for 4% or less based on a total weight of said resin (A),
said resin (B) being substantially free of tetrahydrofuran insolubles and being a polyester resin containing a diol component represented by the formula (1): 
wherein the weight ratio of said resin (A) to said resin (B) is in the range of 60:40 to 85:15.
In another aspect, the present invention provides a toner cartridge containing the above toner.
The present invention also provides an image forming method, comprising developing an electrostatic latent image on an latent image-bearing member with the above toner without using a carrier, said toner being non-magnetic in nature.
The present invention further provides an image forming apparatus comprising a developing unit for developing an electrostatic latent image on an latent image-bearing member with the above toner.